1. Field of the Invention
The present invention relates to a hole transport layer, an organic EL display and a method for manufacturing an organic EL display.
2. Description of Related Art
An organic EL display includes a low molecular organic EL display and a high molecular organic EL display.
The low molecular organic EL display generally includes an anode electrode, a hole injection layer, a hole transport layer, an organic light-emitting layer, an electron transport layer, an electron injection layer, and a cathode layer, which are sequentially stacked on a substrate. The hole transport layer can be removed selectively.
The high molecular organic EL display generally includes an anode electrode, a hole transport layer, an organic light-emitting layer, and a cathode electrode, which are sequentially stacked on a substrate.
The molecular organic EL display forms the layers using a vacuum deposition technique. However, the high molecular organic EL display forms, the layers using a solution processing technique. This limits the formation of the organic light-emitting layer made of an organic material.
In particular, the hole transport layer must have a stable characteristic to a wet coating process to form the organic light-emitting layer using an organic solvent. The hole transport layer is generally made of a water-soluble high molecular weight material such as a mixture of a poly(3,4)-ethylenedioxythiophene (PEDOT) and a polystyrenesulfonate or a mixture of a polyaniline (PANI) and a polystyrenesulfonate. Therefore, there is a problem in that an interface characteristic between the hydrophilic hole transport layer and the hydrophobic organic light-emitting layer is created.
For example, since a bonding power between the hole transport layer and the organic light-emitting layer is low, a life span of the organic EL display is shortened. Also, the organic light-emitting layer cannot be formed on the hydrophilic hole transport layer using an ink-jet technique or a laser induced thermal imaging (LITI) technique, even though the ink-jet printing technique and the LITI technique have an advantage in that it is easy to define pixels and achieve a full color light emission.
U.S. Pat. Nos. 5, 518,824 and 5,922, 481 disclose the hole transport layer including a cross-linkable material. However, the hole transport layer shows a bad interface characteristic between the hole transport layer and the organic light-emitting layer and between the anode electrode and the hole transport layer.